It won't be a 1:1 swap with dark corn syrup and molasses. I'd probably try 3 parts light corn syrup to 1 part molasses; or you could try brown rice syrup, golden syrup (like Lyle's); or cane syrup. I might be wary that the sugar will crystallize differently and could make your pie grainier, however—so it could be a risk.

The molasses is going to give it a very different flavor - seriously not a good idea for a true Southern pecan pie -- wholly apart from the chemical structure.
Yes, brave the grocery store.
BUT, if you have light corn syrup, you can use that, but make sure that at least 2-3 tablespoons of the sugar in the recipe is brown sugar. I've done that many times. (I married a nice fellow from the deep south a long, long time ago and have made dozens of pecan pies for him over the years since, tweaking on occasion along the way . . . . ) Have fun, and Happy Thanksgiving! ;o)

Southern born Virginia Willis is a French-trained chef, food writer, and content creator. Her latest cookbook, Lighten Up, Y'all: Classic Southern Recipes Made Healthy and Wholesome received a James Beard Award of Excellence in 2016.